In present-day vehicles, each seat is provided with a safety belt which is of the type known as a "three point type" safety belt. This means that fixation of the strap that forms the belt is effected as follows. One end of the strap is disposed on an automatic winder that is rigidly connected to the vehicle body and forms the first fixation point. To form the second fixation point, the other end of the belt is fixed by means of a lock or buckle that is placed either on the right side or on the left side of the seat, depending on whether the seat is on the left side or on the right side. The intermediate or third fixation point, i.e. the point situated between the automatic winder and the lock, is normally also fixed to the vehicle body. Thus, the strap that forms the belt is wound on the automatic winder and extends through a keeper situated on the car body at a point thereof disposed substantially at the height of the passenger shoulder and is fixed via its other end, as indicated above, on the lower portion of the vehicle, i.e. on the car body as such, or on the frame of the seating portion of the seat.
However, these known safety belts have a disadvantage of requiring a lengthy installation, and are therefore costly to mount; they are also large (see in particular DE-A-3 429 426 and DE-A-3 613 830).
Attempts have been made to remedy these disadvantages by providing a so-called "three embarked points" safety belt, meaning that the three fixation points of the safety belt are fixed on the frame of the seating portion and of the seat back of the seat. However, one is then confronted with a major disadvantage when, as it is now the case, the inclination of the seat back can be set at will by the passenger. Actually, in this case, the articulation mechanism situated between the seating portion and the seat back are subjected to very large efforts or stresses, which can even lead to breaking of these mechanisms if an accident occurs.
Indeed, the run of the strap that passes over the shoulder and breast of the passenger in order to joint the ventral run of the strap applies, especially when there is an accident, very high stresses on the seat back which can exceed the maximum admissible stress value for the articulation mechanisms.
In addition to this major disadvantage, it has appeared that the presently known winders with automatic blockage were also not able to support such stresses and that therefore, at the moment of an impact, the safety belt does not fulfill absolutely its protection role, which aggravates the risk of bodily harm to the passenger.